Low-Energy Extractive Distillation Process for Dehydration of Aqueous Ethanol

ABSTRACT

An energy-efficient extractive distillation process for producing anhydrous ethanol from aqueous/ethanol feeds containing any range of ethanol employs an extractive distillation column (EDC) that operates under no or greatly reduced liquid reflux conditions. The EDC can be incorporated into an integrated process for producing anhydrous ethanol used for gasoline blending from fermentation broth. By using a high-boiling extractive distillation solvent, no solvent, is entrained by the vapor phase to the EDC overhead stream, even under no liquid reflux conditions. The energy requirement and severity of the EDC can be further improved by limiting ethanol recovery in the EDC. In this partial ethanol recovery design, ethanol which remains in the aqueous stream from the EDC is recovered in a post-distillation column or the aqueous stream is recycled to a front-end pre-distillation column where the ethanol is readily recovered since the VLE curve for ethanol/water is extremely favorable for distillation.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/827,896 that was filed on Jul. 13, 2007.

FIELD OF THE INVENTION

The present invention relates to an improved, energy efficientextractive distillation process for recovering anhydrous (99.5+ wt %)ethanol, that is suitable for gasoline blending, from an aqueous ethanolfeedstock, including fermentation broth.

BACKGROUND OF THE INVENTION

Higher oil prices and more stringent environmental regulations have beenthe impetus for allocating greater resources into research anddevelopment for alternate and renewable fuels worldwide. An importantdevelopment in this regard is the use of ethanol as the blending stockfor gasoline. Burning ethanol instead of gasoline could reduce carbonemissions by more than 80% and completely eliminate the release ofacid-rain-causing sulfur dioxide. The U.S. Department of Energy (DOE)predicts that ethanol could reduce gasoline consumption by 30% in theUnited Sates by 2030. A recent DOE report concluded that, in terms ofkey energy and environmental benefits, fermentation ethanol is clearlysuperior to petroleum-based fuels, and future cellulosic-based ethanolwill be even better.

A major drawback of using ethanol is that fermentation yields diluteaqueous ethanol mixtures that contain only 10-12 wt % ethanol. Currentdehydration strategies, for producing anhydrous (99.5 wt %) ethanol thatis suitable for gasoline blending, are energy intensive and employconventional distillation and a subsequent step to break theethanol-water azeotrope. As compared to the energy value of ethanolwhich is 21,400 KJ/L, conventional distillation requires approximately6,400 KJ/L to concentrate fermentation ethanol to 95.6 wt % ethanol,which is the azeotropic composition. Even improved distillation schemesthat employ heat integration still require 5,500 KJ/L to produce theazeotropic composition. Thereafter, concentrating the ethanol from theazeotropic point to produce anhydrous ethanol requires more energy. Asis apparent, despite the benefits of using ethanol, the energy costsassociated with dehydrating ethanol present serious economic impedimentsfor using ethanol produced by fermentation as a gasoline blending stockor as an engine fuel.

The major commercial methods for concentrating ethanol beyond theazeotropic composition for producing anhydrous ethanol are: (1)azeotropic distillation, (2) molecular sieves adsorption, and (3)extractive distillation. A fourth method known as membrane separationwhich uses zeolitic or polymer membranes to break the azeotrope islargely in the developmental stages.

In azeotropic distillation, a volatile entrainer is added to an aqueousethanol feed mixture. The entrainer modifies the activity coefficientsof the water and ethanol being separated and forms an azeotrope with thewater to be taken overhead as the process yields anhydrous ethanol.Pentane, benzene, diethyl ether, and gasoline have been disclosed assuitable entrainers. See, U.S. Pat. No. 3,575,818 to West, U.S. Pat. No.2,012,199 to McElroy, U.S. Pat. No. 2,371,010 to Wolfner, and Black etal., “Extractive and Azeotropic Distillation,” Am. Chem. Soc. AdvancesIn Chemistry Series No. 115, p. 64, 1972. The major disadvantage of theazeotropic method is that the ethanol feed has to be pre-concentrated tonear 95 wt %, the azeotropic composition, which is an energy intensiveprocess in itself.

Adsorptive separation processes for separating of ethanol from water aretypically batch processes, with respect to the adsorbents used, thatentail an adsorption cycle and a separate desorption cycle. Variousadsorptive separation techniques are described in, for example, U.S.Pat. No. 4,273,621 to Formoff, U.S. Pat. No. 4,407,662 to Ginder, U.S.Pat. No. 4,465,875 to Greenbank et al., U.S. Pat. No. 4,287,089 toConvers et al., U.S. Pat. No. 4,277,635 to Oulman, U.S. Pat. No.4,382,001 to Kulprathipanja et al., U.S. Pat. No. 5,030,775 to Sircar,U.S. Pat. No. 4,343,623 to Kulprathipanja et al, U.S. Pat. No. 4,319,058to Kulprathipanja et al, U.S. Pat. No. 5,766,895 to Valkanas et al, U.S.Pat. No. 4,359,593 to Feldman and U.S. Pat. No. 2,137,605 to Derr.Although adsorption methods are very selective in removing water orethanol, the associated high heat energy requirements, high operatingcosts, limited capacities, and uncertainty in the lengths of adsorbentlives are major drawbacks toward commercial operations.

Finally, in extractive distillation (ED) a high-boiling, polar,nonvolatile solvent is added to the upper portion of an extractivedistillation column (EDC), while the feed containing ethanol and wateris fed the middle or lower portion of the EDC, which is below thesolvent entry point. Depending upon the properties of the solvent, thedescending nonvolatile solvent preferentially extracts water or ethanolfrom the ascending vapor stream thereby eliminating the ethanol-waterazeotrope and producing purified ethanol or water from the overhead ofEDC. A portion of the EDC overhead stream is recycled to the top of theEDC as reflux. Saturated solvent that is rich in water or ethanol isthen withdrawn from the bottom of EDC and transferred to the middleportion of a solvent recovery column (SRC). Water or ethanol in thesaturated solvent is stripped from the solvent by heat from a SRCreboiler and then recovered from the overhead stream of the SRC aspurified water or ethanol. Again, a portion of the overhead stream isrecycled to the top of SRC as liquid reflux. Lean solvent from thebottom of SRC is circulated back to the EDC as the solvent feed.

An ED process for dehydrating aqueous ethanol using glycerin as the EDsolvent was disclosed in U.S. Pat. No. 1,469,447 to Schneible.Subsequently, other ED solvents considered include: ethoxyethanol andbutoxyethanol (U.S. Pat. No. 2,559,519 to Smith et al.), butyl, amyl orhexyl alcohols (U.S. Pat. No. 2,591,671 to Catterall), gasolinecomponents (U.S. Pat. No. 2,591,672 to Catterall), sulfuric acid,acetone or furfural (U.S. Pat. No. 2,901,404 to Kirshenbaum et al.),2-phenyl phenol, or mixtures of 2-phenyl phenol and cumyl phenol (U.S.Pat. No. 4,428,798 Zudkevitch et al.), cyclohexylcyclohexanone orcyclohexylcyclohexanol (U.S. Pat. No. 4,455,198 to Zudkevitch et al.),methyl benzoate, mixture of methyl benzoate and trimellitic anhydride,and mixture of dipropylene glycol dibenzoate, ethyl salicylate andresorcinaol (U.S. Pat. No. 4,631,115 to Berg et al.), hexahydrophthalicanhydride, mixture of methyl tetrahydrophthalic anhydride andpentanol-1, and mixture of trimellitic anhydride, ethyl salicylate andresorcinol (U.S. Pat. No. 4,654,123 to Berg et al.); and diaminobutane,1,3-diaminopentane, diethylenetriamine, and hexachlorobutadiene (U.S.Pat. No. 6,375,807 to Nieuwoudt). The ED solvents disclosed in thesepatents were said to be particularly selective in separating ethanol andwater, but without regard to their practical applicability to EDprocesses with respect to other important solvent properties such assolvent thermal stability, toxicity, boiling point, etc. These patentsalso did not address energy and related economics issues related to EDprocesses.

In order to reduce the energy requirements in ED processes, U.S. Pat.No. 4,400,241 to Braithwaite et al. proposed adding alkali-metal oralkaline-earth metal salts to a polyhydric alcohol solvent to enhancethe ED solvent performance. The preferred systems include (1) sodiumtetraborate that is added to ethylene glycol and (2) dipotassiumphosphate that is added glycerin.

Other approaches to reducing energy requirements featured improvedprocess designs to recovery energy such as that in U.S. Pat. No.4,349,416 to Brandt et al. where a first side stream is withdrawn fromthe EDC, passed in heat exchange with the bottoms from the EDC en routeto the SRC and returned to the EDC at a point below the point of theside stream. A second side stream from the EDC is also withdrawn, passedin heat exchange with the bottoms of the SRC and returned to the EDC.

U.S. Pat. No. 4,559,109 to Lee et al. disclosed another approach wherebyaqueous ethanol containing 10-12 wt % ethanol is first converted to an85-90 wt % concentrated vapor in a front-end distillation column whichis then fed to an EDC. The front-end distillation column represents thewater-rich (lower) portion of the fractionator, while the EDC representsthe ethanol-rich (upper) portion of the fractionator. Extractive solventis added only to the EDC (ethanol-rich portion of the fractionator)where the vapor-liquid equilibrium (VLE) curve is very unfavorable fordistillation. The solvent is said to eliminate the binary ethanol-waterazeotrope and modify the shape of the ethanol-rich portion of the VLEcurves favorably for distillation. The ethanol-water saturated solventis then completely removed via a rich solvent bottoms stream from theEDC and fed to a SRC (solvent stripper). The vaporous overhead stream ofthe SRC is said to be recycled directly to an upper portion of thefront-end distillation column while lean solvent from the bottom of theSRC is fed to the top of the EDC. In a preferred application of theprocess, the rich solvent bottoms stream from the EDC which is fed tothe SRC contains more than 40 wt % of the ethanol that is present in thevaporous feed to the EDC from the front-end distillation column (orsimply more than 40 wt % of the ethanol in the feed to the front-enddistillation column).

In practice the techniques described in U.S. Pat. No. 4,559,109 areneither energy nor cost effective because the rich solvent stream, whichcontains 40 wt % of the feed ethanol, must be vaporized in the SRC andthen again in the pre-distillation column. Initially the rich solvent isvaporized in the SRC to separate the solvent from the water and theethanol, but it has been demonstrated that the water and the ethanolcannot be directly recycled to the pre-distillation column in the formof vapor as stated in the patent. The reason is that the SRC is normallyoperated under reduced pressures in order to lower the bottomstemperature so as to minimize solvent decomposition. When ethyleneglycol is employed as the ED solvent, as shown in FIGS. 5 and 6 of thepatent, the SRC should be operated at substantially lowered pressures ofabout 150 mmHg at the top in order to keep the bottoms temperature below180° C. The pre-distillation column, on the other hand, is operated athigher pressures than that of the EDC to allow the vapor stream to befed from the overhead of the pre-distillation column to the EDC, whichis preferred procedure as described in the patent. Because of theseconstraints, the overhead vapor stream of the SRC has to be condensedbefore it can be pumped into the higher pressured pre-distillationcolumn, where the ethanol content in the condensed SRC recycle stream isre-vaporized. Another problem associated with the process depicted inU.S. Pat. No. 4,559,109 is that there is no liquid reflux for the SRCwhich causes the upper trays in the SCR to run dry.

Other energy saving ED techniques employ a combination of the basicdistillation method along with (1) multi-effect distillation, (2)conventional overhead-to-reboiler heat pumped distillation, (3)azeotropic and extractive distillation, or (4) fermentative productionof volatile compounds, which is disclosed in U.S. Pat. No. 4,961,826 toGrethlein et al. All of the energy saving methods in the prior art forthe ED process or combination process schemes associated ED methods areeither too limited in scope for energy savings or too complicated forpractical applications.

SUMMARY OF THE INVENTION

The present invention provides a simple, energy efficient extractivedistillation (ED) process for producing anhydrous ethanol (99.5+ wt %ethanol) from aqueous ethanol feeds containing any range of ethanol. Theimproved ED technique is particularly suited for incorporation into anintegrated process for producing anhydrous ethanol, which is suitablefor gasoline blending, from fermentation broth which typically contains1 to 12 wt % and more preferably 10 to 12 wt % ethanol.

The invention is based in part on the discovery that the liquid refluxwithin an extractive distillation column (EDC) has essentially no effectwith respect to separating ethanol and water; rather, when liquid refluxis employed, it functions primarily to knock down the entrained solventwhich is otherwise carried by the vapor phase into the EDC overheadstream. A corollary to operating the EDC under no liquid reflux orgreatly reduced liquid reflux conditions is that the energy input to theEDC is substantially reduced. An aspect of the invention is that byusing a high-boiling ED solvent, no solvent is entrained by the vaporphase to the EDC overhead stream, even under no liquid refluxconditions. A feature of the invention is that the energy requirementand severity of the EDC can be further improved by modifying the processto limit ethanol recovery in the EDC. In this partial ethanol recoverydesign, ethanol which remains in the aqueous stream from the EDC isrecovered in a post-distillation column or the aqueous stream isrecycled to a front-end pre-distillation column where the ethanol isreadily recovered since the water-rich portion of the vapor-liquidequilibrium curve for ethanol/water is extremely favorable fordistillation.

In one embodiment, the invention is directed to an improved extractivedistillation (ED) process for dehydrating an aqueous feedstockcontaining ethanol and water that includes the steps of:

-   -   (a) introducing an aqueous feedstock comprising ethanol and        water into a middle portion of an extractive distillation column        (EDC);    -   (b) introducing a high-boiling water selective solvent into an        upper portion of the EDC to contact the aqueous feedstock under        extractive distillation conditions to produce a liquid bottoms        stream that comprises water and high-boiling water selective        solvent and to produce a vaporous overhead stream that comprises        greater than 99.5 weight percent ethanol, wherein the EDC is        operated with a liquid reflux-to-distillate ratio of less than        about 0.5;    -   (c) withdrawing at least a portion of the vaporous overhead from        the EDC as a purified ethanol product;    -   (d) feeding at least a portion of the liquid bottoms stream of        the EDC into a solvent recovery column (SRC) to remove water        therefrom and to yield a lean high-boiling water selective        solvent stream; and    -   (e) recycling at least a portion of the lean high-boiling water        selective solvent stream into an upper portion of the EDC.

In another embodiment, the invention is directed to an improvedextractive distillation (ED) process for dehydrating an aqueousfeedstock containing ethanol and water that includes the steps of:

-   -   (a) introducing an aqueous feedstock comprising ethanol and        water into a middle portion of an extractive distillation column        (EDC);    -   (b) introducing a high-boiling ethanol selective solvent into an        upper portion of the EDC to contact the aqueous feedstock under        extractive distillation conditions to produce a liquid bottoms        stream consisting essentially of ethanol and high-boiling        selective solvent and a vaporous overhead stream comprising        essentially of water, wherein the EDC is operated with a liquid        reflux-to-distillate ratio of less than about 0.5;    -   (c) feeding at least a portion of the liquid bottoms stream of        the EDC into a solvent recovery column (SRC) to remove ethanol        therefrom and to yield a bottoms stream that comprises lean        high-boiling ethanol selective solvent and an overhead stream        whereby at least a portion of the overhead is withdrawn as a        purified ethanol product with higher than 99.5 wt % purity; and    -   (d) recycling at least a portion of the lean high-boiling        ethanol selective solvent from the bottoms stream of the SRC        into the upper portion of the EDC.

In a further embodiment, the invention is directed to an improvedprocess for dehydrating an aqueous feedstock containing ethanol andwater that includes the steps of:

-   -   (a) distilling an aqueous feedstock comprising ethanol and water        in a pre-distillation column to produce a first vaporous        overhead stream, a vaporous side-cut stream, and a first liquid        bottoms stream;    -   (b) partially condensing the first vaporous overhead stream in a        first condenser to yield a condensate phase and a vapor phase;    -   (c) recycling the condensate phase from the first condenser into        the pre-distillation column as reflux;    -   (d) introducing the vaporous side-cut stream into a middle        portion of an extractive distillation column (EDC);    -   (e) introducing a bottoms stream from a solvent recovery column        (SRC) which contains a high-boiling water selective solvent into        an upper portion of the EDC to contact the vaporous side-cut        stream under extractive distillation conditions to produce a        second vaporous overhead stream that comprises greater than 99.5        wt % ethanol and a second liquid bottoms stream comprising        water, ethanol and high-boiling water selective solvent wherein        the EDC is operated with a liquid reflux-to-distillate (RID)        ratio of less than about 0.5;    -   (f) introducing at least a portion of the second liquid bottoms        stream from the EDC that comprises water, ethanol and        high-boiling water selective solvent into the SRC to remove        water and ethanol from the high-boiling water selective solvent;    -   (g) recycling at least a portion of a bottoms stream of the SRC        that comprises high-boiling water selective solvent into an        upper portion of the EDC;    -   (h) recycling a first portion of an overhead condensate from the        SRC as liquid reflux to the SRC and recycling a second portion        of the overhead condensate into the pre-distillation column; and    -   (i) withdrawing a bottom stream from the pre-distillation column        that consists essentially of water.

In yet another embodiment, the invention is directed to an improvedprocess for dehydrating an aqueous feedstock containing ethanol andwater that includes the steps of

-   -   (a) distilling an aqueous feedstock comprising ethanol and water        in a pre-distillation column to produce a first vaporous        overhead stream, a vaporous side-cut stream, and a first liquid        bottoms stream;    -   (b) partially condensing the first vaporous overhead stream in a        first condenser to yield a condensate phase and a vapor phase;    -   (c) recycling the condensate phase from the first condenser into        the pre-distillation column as reflux;    -   (d) introducing the vaporous side-cut stream into a middle        portion of an extractive distillation column (EDC);    -   (e) introducing a bottoms stream from a solvent recovery column        (SRC) which contains a high-boiling ethanol selective solvent        into an upper portion of the EDC to contact the vaporous        side-cut stream under extractive distillation conditions to        produce a second vaporous overhead stream that comprises water        and ethanol and a second liquid bottom streams that comprises        ethanol and the high-boiling ethanol selective solvent wherein        the EDC is operated with a liquid reflux-to-distillate ratio of        less than about 0.5;    -   (f) recycling at least a portion of the second vaporous overhead        stream from the EDC in the form of condensate into the        pre-distillation column;    -   (g) introducing at least a portion of the second liquid bottoms        stream from the EDC that comprises ethanol and high-boiling        ethanol selective solvent into the SRC to yield an overhead        ethanol product that comprises greater than 99.5 weight percent        ethanol and a bottoms stream that comprises high-boiling ethanol        selective solvent;    -   (h) recycling at least a portion of the SRC bottoms stream        comprising the high-boiling ethanol selective solvent into an        upper portion of the EDC;    -   (i) recycling a portion of the ethanol product in the form of        condensate into the SRC as reflux; and    -   (j) withdrawing a bottoms stream from the pre-distillation        column that consists essentially of water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a low-energy extractive distillation process for thedehydration of aqueous ethanol;

FIGS. 2 and 3 illustrate extractive distillation processes having apost-distillation column for dehydrating aqueous ethanol usinghigh-boiling water selective solvents and using high-boiling ethanolselective solvents, respectively; and

FIGS. 4 and 5 illustrate extractive distillation processes havingpre-distillation column for dehydrating fermentation broth usinghigh-boiling water selective solvents and high-boiling ethanol selectivesolvents, respectively.

DETAILED DESCRIPTION OF THE INVENTION

In conventional distillation, the liquid reflux at the top ofdistillation column governs the amount of liquid phase that is inequilibrium with the rising vapor phase at each of the contacting stagesin the column. Depending on the feed composition, the productrequirements, and the vapor liquid equilibrium curve (VLE) of the keycomponents being separated, the reflux to distillate (R/D) ratio mayrange from 1 to 10 or even higher and typically ranges from 2 to 5.Therefore, the R/D ratio is the major operating variable that determinesnot only the product purity and recovery but the process energyrequirements as well since the liquid reflux has to be vaporized in thecolumn.

The present invention is based, in part, on the discovery that, inextractive distillation (ED), the ED solvent that is fed to the upperportion (near the top) of an extraction distillation column (EDC) canserve as the liquid phase as it descends down the column and is inequilibrium with the ascending vapor phase at each stage in the column.Hence, the liquid reflux which dictates the major energy requirement ofthe distillation column has essentially no functional purpose in theEDC.

Low-Energy Extractive Distillation Scheme for Dehydration of AqueousEthanol

Referring to the process shown in FIG. 1, an aqueous ethanol feed is fedvia line or stream 1 to the middle portion of extractive distillationcolumn (EDC) 201. The feed contains typically 10 to 95 wt % ethanol,preferably 85 to 95 wt % ethanol, and more preferably 90 to 92 wt %ethanol. In a preferred but optional embodiment, feed stream 1 isinitially heated by the EDC overhead vapor in condenser 204 before beingpartial or totally vaporized by the lean solvent feed to the EDC incooler 203. Lean solvent from the bottom of solvent recovery column(SRC) 202 is fed via lines 13 and 2 into an upper portion of EDC 201after being cooled in cooler 203. The lean solvent entry point in EDC201 is selected at a location such that there are a few trays above thesolvent tray to essentially eliminate entrained solvent that is carriedover to the EDC overhead. Overhead vapor exiting EDC 201 via line 3 iscondensed in condenser 204 and the condensate is transferred intoaccumulator 206. Preferably, no liquid reflux is recycled to the top ofEDC 201 although a reflux-to-distillate (R/D) ratio ranging from 0 toless than 0.5 can be used for non-functional purposes, such as forknocking down entrained solvent from the overhead vapor stream 3. Ifnecessary, the lean solvent temperature can be maintained 20° C. andpreferably 10° C. below the EDC temperature near the solvent tray inorder to generate internal reflux without expending energy. If thehigh-boiling ED solvent that is used preferentially extracts water inEDC 201, ethanol product with 99.5+ wt % purity is withdrawn fromaccumulator 206 by pump 208 through line 4. On the other hand, if thehigh-boiling ED solvent that is used preferentially extracts ethanol inEDC 201, a stream consisting essentially of water is withdrawn fromaccumulator 206 by pump 208 through line 4. To ensure that the overheadstream 3 is not contaminated with entrained solvent when the system isoperating under no reflux conditions, the preferred high-boiling EDwater selective solvent that is employed for preferentially extractingwater in EDC 201 preferably has a boiling point of at least 200° C. andis selected from glycerin, ethylene glycol, diethylene glycol,triethylene glycol, tetraethylene glycol, trimethylene glycol, 1,4butanediol, and combinations thereof. On the basis of their boilingpoints and selectivity, the preferred solvents are glycerin,tetraethylene glycol, and ethylene glycol, and diethylene glycol.

Preferred high boiling ED ethanol selective solvent for preferentiallyextracting ethanol in the EDC preferably has a boiling point of at least200° C. and is selected from of C₆+ phenols (including 2-phenyl-phenol,cumyl phenol, diisopropyl phenol and mixtures thereof), cyclic C₇ketones (including cyclohexyl cyclohexanone), cyclic C₈ alcohols(including cyclohexyl cyclohexanol), methyl benzoate, dipropylene glycoldibenzoate, trimellitic anhydride, and mixtures thereof. Solvents thatcontain sulfur should not be used.

To keep the bottom temperature of EDC 201 at less than 200° C. andpreferably in the range of 160° C. to 180° C. in order to minimizesolvent decomposition, EDC 201 is operated under slightly positivepressures. Depending upon the solvent function for extracting water orethanol, rich solvent saturated with ethanol or water, respectively, iswithdrawn from the bottom of EDC 201 with pump 211 via line 5. A portionof the bottoms is passed through reboiler 210 and recycled to the bottomof EDC 201 via line 6 to maintain the vapor flow in the column. Sincethere is minimal or no reflux in EDC 201, the energy requirement forreboiler 210 is substantially reduced.

Rich solvent stream from EDC 201 in the form of a partially vaporizedmixture is fed via lines 5 and 7 to the middle portion of solventrecovery column (SRC) 202, which is operated at reduced pressures(vacuum) to maintain the bottom temperature at below 200° C. andpreferably in the range of 160° C. to 180° C. in order to minimizesolvent decomposition. A vapor stream containing essentially pure wateror ethanol, depending upon the solvent function, exits the top of SRC202 via line 8 and is condensed in condenser 205. Pressure in condenser205 as well as the top of SRC 202 typically ranges from 50 to 500 mmHg(absolute) and preferably from 150 to 300 mmHg (absolute), dependingupon the boiling point of the solvent. The SRC 202 overhead is connectedto a vacuum source (not shown), which can be a vacuum pump or steamejector, through line 9. The condensate from condenser 205 istransferred via line 10 to accumulator 207, where a portion of thecondensate is recycled through pump 209 to the top of SRC 202 via line11 as a liquid reflux to provide liquid flow for the upper portion ofSRC 202. The remaining portion of the condensate is withdrawn fromaccumulator 207 with pump 209 via line 12 in the form of water or theanhydrous ethanol product, again, depending on the solvent function.

In the meantime, lean solvent is withdrawn from the bottom of SRC 202with pump 213 via line 13. A portion of this lean solvent is heated inreboiler 212 and recycled to the bottom of SRC 202 via line 14 tomaintain the bottoms temperature in the desired temperature range. Inthe case where the anhydrous ethanol product is produced from theoverhead of EDC 201, maintaining the bottoms temperature within theseranges keeps the water content in the lean solvent below 1 wt % andpreferably in the range of 0.2 to 0.5 wt %. Higher water content in thelean solvent would lower the ethanol product purity since the leansolvent is fed to near the top of EDC 201.

Extractive Distillation Scheme with a Post-Distillation Column forDehydration of Aqueous Ethanol Using Water Selective Solvents

FIG. 2 shows an integrated process for dehydrating aqueous ethanol thatemploys a system which includes an EDC 221, SRC 222, and apost-distillation column (Post-DC) 223 wherein the ED solvent usedpreferentially extracts water in the EDC. Again, aqueous ethanol feedcontaining 10 to 95 wt % ethanol, preferably 85 to 95 wt % ethanol, andmore preferably 90 to 92 wt % ethanol is fed into EDC 221 via lines orstreams 21 and 22. In a preferred but optional embodiment, feed stream21 is initially heated by the EDC overhead vapor which is circulatedthrough condenser 225 before being partial or totally vaporized by thelean solvent feed that is circulated through cooler 224.

Lean solvent from the bottom of SRC 222 is fed via lines 33 and 23 intoan upper portion of EDC 221 after being cooled in cooler 224. The leansolvent entry point in EDC 221 is selected at a location such that thereare a few trays above the solvent tray to essentially eliminateentrained solvent that is carried over to the EDC overhead. Overheadvapor exiting EDC 221 via line 24 is condensed in condenser 225 and thecondensate is transferred into accumulator 228. Preferably, no liquidreflux is recycled to the top of EDC 221 although a reflux-to-distillateratio ranging from 0 to less than 0.5 can be used for non-functionalpurposes, such as for knocking down entrained solvent from the overheadvapor stream 24. If necessary, the lean solvent temperature can bemaintained 20° C. and preferably 10° C. below the EDC temperature nearthe solvent tray in order to generate internal reflux without expendingenergy. Ethanol product with 99.5+ wt % purity is withdrawn fromaccumulator 228 by pump 231 through line 25.

To keep the bottom temperature of EDC 221 at less than 200° C. andpreferably at less than 180° C. in order to minimize solventdecomposition, EDC 221 is operated under slightly positive pressures.Rich solvent saturated with water is withdrawn from the bottom of EDC221 with pump 237 via line 26. A portion of the bottoms is passedthrough reboiler 234 and recycled to the bottom of EDC 221 via line 27to maintain the vapor flow in the column.

In this process, EDC 221 is configured to produce an ethanol productwith 99.5+ wt % purity in the overhead and to allow a certain amount ofethanol in the bottoms. EDC 221 operates without liquid reflux (orminimal reflux) and requires fewer separation stages and a lowersolvent-to-feed (S/F) ratio. The EDC bottoms contains preferably no morethan 20 wt % and more preferably no more than 5 wt % of the ethanol thatis in the feed to EDC 221. The preferred operating conditions of EDC 221and SRC 222 are similar to those disclosed for EDC 201 and SRC 202,respectively, for the process shown in FIG. 1.

Rich solvent stream from EDC 221 in the form of a partially vaporizedmixture is fed via lines 26 and 28 to the middle portion of SRC 222,which is operated at reduced pressures (vacuum) to maintain the bottomtemperature at below 200° C. and preferably in the range of 160° C. to180° C. in order to minimize solvent decomposition. Overhead aqueousvapor stream from SRC 222, which contains water and a certain amount ofethanol, is transferred via line 29 to condenser 226 under reducedpressure (vacuum), where the condensate is transferred to accumulator229 via line 31. SRC 222 overhead is connected to a vacuum source (notshown) such as a vacuum pump or steam injector through line 30. At leasta portion of the condensate is recycled to SRC 222 as liquid reflux bypump 232 via line 32. The remaining portion of the condensate is fed toPost-DC 223 via line 35 to recover ethanol. Based on VLE curves forwater and ethanol mixtures, Post-DC 223 can be operated favorably toproduce pure water in the bottoms and a vapor mixture of water andethanol in the overhead stream, which exits via line 37; at least aportion of the overhead vapor in line 38 is condensed in condenser 227and sent to accumulator 230 via line 40 and then recycled to Post-DC 223as liquid reflux through pump 233. In order to recycle the remainingportion of overhead vapor stream from Post-DC 223 to EDC 221 via lines39 and 22 to recover ethanol, Post-DC 223 is operated at higherpressures than that of EDC 221. The bottoms stream of Post-DC 223containing pure water is withdrawn by pump 239 via line 36. A portion ofthis bottoms stream is heated by reboiler 236 and recycled to the bottomof Post-DC 223 for generating the vapor flow whereas the rest of stream36 is withdrawn via line 41 for disposal.

In the meantime, lean solvent is withdrawn from the bottom of SRC 222with pump 238 via line 33. A portion of this lean solvent is heated inreboiler 235 and recycled to the bottom of SRC 222 via line 34 tomaintain the bottoms temperature in the desired temperature range.

Extractive Distillation Scheme with a Post-Distillation Column forDehydration of Aqueous Ethanol Using Ethanol Selective Solvents

FIG. 3 shows an integrated process for dehydrating aqueous ethanol in asystem that includes an EDC 241, SRC 243, and a Post-DC 242 wherein theED solvent used preferentially extracts ethanol in EDC 241. An aqueousethanol feed is fed via lines or streams 51 and 52 to the middle portionof extractive distillation column 241. The feed contains typically 10 to95 wt % ethanol, preferably 85 to 95 wt % ethanol, and more preferably90 to 92 wt % ethanol. In a preferred but optional embodiment, feedstream 51 is partial or totally vaporized by the lean solvent feed toEDC 241 in cooler 244. Lean solvent from the bottom of SRC 243 is fedvia lines 62 and 53 into an upper portion of EDC 241 after being cooledin cooler 244. The lean solvent entry point in EDC 241 is selected at alocation such that there are a few trays above the solvent tray toessentially eliminate entrained solvent that is carried over to the EDCoverhead.

In this scheme, EDC 241 is configured to produce a rich solventcontaining only solvent and ethanol in the bottom of EDC 241 and toallow a certain amount of ethanol in the overhead aqueous stream so thatEDC 241 can be operated without liquid reflux (or minimal flux) andunder much less demanding conditions of lower S/F and fewer separationstages. The overhead stream from EDC 241 contains preferably no morethan 20 wt % and more preferably no more than 5 wt % of the ethanol inthe feed to EDC 241. Rich solvent saturated with ethanol is withdrawnfrom the bottom of EDC 241 with pump 254 via line 55. A portion of thebottoms is passed through reboiler 251 and recycled to the bottom of EDC241 via line 56 to maintain the vapor flow in the column. The preferredoperating conditions of EDC 241 are similar to those described for EDC201 in the process shown in FIG. 1.

The overhead aqueous vapor stream from EDC 241 containing water and acertain amount of ethanol is fed via line 54 to Post-DC 242. Again,Post-DC 242 can be operated favorably to produce pure water in thebottoms and a vapor mixture of water and ethanol in the overhead stream,which exits via line 64 and is condensed in condenser 245 andtransferred to accumulator 247 via line 65. At least a portion of thecondensate is recycled to Post-DC 242 as liquid reflux via line 66 bypump 249. The remaining portion of the condensate is recycled to EDC 241via lines 67 and 52 to recover the ethanol. The bottoms stream ofPost-DC 242 containing pure water is withdrawn by pump 255 via line 68and a portion is disposed of through line 70 and the remaining portionis heated in reboiler 252 before being recycled back into Post-DC 242via line 69. In order to feed the overhead vapor of EDC 241 into columnPost-DC 242, EDC 241 is operated at higher pressures than that ofPost-DC 242. EDC 241 bottom (rich solvent) stream containing onlyethanol and solvent is withdrawn by pump 254 and fed into SRC 243 vialines 55 and 57. The preferred operating conditions of SRC 243 aresimilar to those disclosed in the description of SRC 202 in FIG. 1, tomaintain the bottom temperature at below 200°C. and preferably in therange of 160° C. to 180° C. to minimize solvent decomposition.

A vapor stream containing essentially of pure ethanol exits the top ofSRC 243 via line 58 and is condensed in condenser 246. The pressure incondenser 246 as well as at the top of SRC 243 ranges from 50 to 500mmHg (absolute) and preferably from 150 to 300 mmHg (absolute),depending upon the boiling point of the solvent. SRC 243 overhead isconnected to a vacuum source (not shown), which can be a vacuum pump orsteam ejector, through line 71. The condensate from condenser 246 istransferred via line 59 to accumulator 248, where a portion of thecondensate is recycled through pump 250 to the top of SRC 243 via line60 as a liquid reflux to provide liquid flow for the upper portion ofSRC 243. The remaining portion of the condensate is withdrawn fromaccumulator 248 with pump 250 via line 61 in the form of anhydrousethanol.

In the meantime, lean solvent is withdrawn from the bottom of SRC 243with pump 256 via line 62. A portion of this lean solvent is heated inreboiler 253 and recycled to the bottom of SRC 243 via line 63 tomaintain the bottoms temperature in the desired temperature range.

Extractive Distillation Scheme with a Pre-Distillation Column forDehydration of Fermentation Broth Using Water Selective Solvents

FIG. 4 shows an integrated process for dehydration of fermentation brothwhere the ED solvent used preferentially extracts water in the EDC. Thesystem includes a Pre-Distillation Column (Pre-DC) 261, EDC 262 and SRC263. Fermentation broth, which typically contains 10 to 12 wt % ethanol,is fed via lines 81 and 82 into the middle portion of Pre-DC 261. In apreferred but optional embodiment, feed stream 81 is initially heated bythe EDC overhead vapor that is circulated into condenser 267 andsubsequently heated by the energy from the bottoms water stream fromPre-DC 261 in heat exchanger 264 before being partial or totallyvaporized by the lean solvent feed to the EDC through heat exchanger266. A vaporous side-cut stream containing 80 to 95 wt % ethanol,preferably 85 to 92 wt % ethanol, and more preferably 90 to 92 wt %ethanol is withdrawn from Pre-DC 261 from a location in Pre-DC 261 thatis preferably 1 or 2 trays below the top and just below the liquidreflux entry point and the vapor fed into EDC 262 as the vaporousethanol feed. The overhead stream in line 83 comprising vaporousethanol, water and essentially all the light minor components includingformaldehyde, methanol, and light esters from the top of Pre-DC 261passes through partial condenser 265, which is maintained at an averagetemperature that is from 60 to 65° C. At this temperature range, onlyethanol and water are condensed and the condensate from accumulator 269is recycled with pump 272 into Pre-DC 261 as liquid reflux via line 84.The vaporous light components, which are not suitable for humanconsumption, exit condenser 265 via line 86 and are subsequentlycondensed separately at lower temperatures.

By using glycerin or other nontoxic ED solvents, food and medicinalgrade ethanol can be produced with this process. The bottoms stream fromPre-DC 261 which contains essentially water is withdrawn via line 87 bypump 278. A portion of this bottoms stream is heated in reboiler 275 andrecycled into the bottom of Pre-DC 261 via line 88 whereas the remainingportion of the bottom stream is passed through heat exchanger 264 toheat the aqueous ethanol feed to Pre-DC 261 and the cooled bottomsstream is sent back to a fermentation section (not shown) via line 89.By designing the process so that the side-cut at the top of Pre-DC 261contains no more than 92 wt % ethanol, the liquid reflux and number oftrays in Pre-DC 261 are substantially reduced. It is estimated thatPre-DC 261 requires approximately 2,570 KJ/L of energy as compared to4,000 KJ/L (a 36% increase) that is required when the side cut streamcontains 95 wt % ethanol.

A vaporous side-cut from Pre-DC 261 is fed via line 85 to the middleportion of EDC 262 whereas lean solvent from the bottom of SRC 263 isfed by pump 280 via lines 101 and 90 to the upper portion of EDC 262after being cooled in heat exchanger 266. The lean solvent entry pointin EDC 262 is selected so that there are a few trays above the solventtray to essentially eliminate the presence of entrained solvent, if any,from being carried over to the overhead of EDC 262. Preferably, noliquid reflux is recycled back to the top of EDC 262 although areflux-to-distillate ratio ranging from 0 to less than 0.5 can be usedfor non-functional purposes, such as for knocking down any entrainedsolvent from the overhead vapor stream. If necessary, the lean solventtemperature can be maintained at 20° C. and preferably 10° C. below thecolumn temperature near the solvent tray which is sufficient to generateinternal reflux without additional energy input. Overhead vapor exitingthe EDC 262 via line 91 is condensed in condenser 267 and thentransferred into accumulator 270. An ethanol product that is 99.5+ wt %pure is withdrawn from accumulator 270 with pump 273 through line 92.

In order to achieve a 99+ wt % ethanol recovery with 99.5+ wt % purityEDC 262 must be designed with a large number of separation trays and thecolumn would need to operate under a high solvent-to-feed ratio. Tominimize capital expenditure and reduce energy requirements,alternatively, EDC 262 can be designed and operated under conditions toachieve less than total ethanol recovery. In particular, in a preferredembodiment, no more than 20 wt %, and preferably no more than 5 wt %, ofthe ethanol that is in the EDC feed reaches the EDC bottoms (richsolvent) stream.

The rich solvent, which contains preferably no more than 5 wt % of theethanol in the EDC feed, is withdrawn from the bottom of EDC 262 and isfed via lines 93 and 95 by pump 279 as a partially vaporized mixtureinto the middle portion of SRC 263. A portion of the EDC bottoms isheated through reboiler 276 and recycled to the bottom of EDC 262 vialine 94 to maintain the vapor flow in EDC 262. The bottoms temperatureof EDC 262 preferably is in the range from 160° C. to 200° C. Leansolvent is withdrawn from the bottoms of SRC 263 with pump 280 via line101. A portion of this lean solvent is heated in reboiler 277 andrecycled to the bottom of SRC 263 via line 102 to maintain the bottomstemperature in the range of 160° C. to 200° C. and preferably in therange of 160 to 180° C. SRC 263 typically has an operating pressure inthe range of 50 to 500 mm Hg (absolute) and preferably in the range of150 to 300 mm Hg (absolute). Overhead aqueous vapor stream from SRC 263is transferred via line 96 to condenser 268 under reduced pressure(vacuum), where the condensate is transferred to accumulator 271 vialine 97. SRC 263 overhead is connected to a vacuum source (not shown)such as a vacuum pump or steam injector through line 98. At least aportion of the condensate is recycled to SRC 263 as liquid reflux bypump 274 via line 99. The remaining portion of the condensate isrecycled to Pre-DC 261 via lines 100 and 82 to recover the ethanol.

Extractive Distillation Scheme with a Pre-Distillation Column forDehydration of Fermentation Broth Using Ethanol Selective Solvents

FIG. 5 shows an integrated process for dehydration of the fermentationbroth where the ED solvent used preferentially extracts ethanol in theEDC. The system includes Pre-DC 281, EDC 282, and SRC 283. Fermentationbroth containing 10 to 12 wt % ethanol is fed via lines 111 and 112 intothe middle portion of Pre-DC 281. In a preferred but optionalembodiment, feed stream 111 is initially heated by the bottom waterstream from Pre-DC 281 in heat exchanger 284 and feed stream 111 is thenheated by the EDC overhead vapor which is sent through condenser 287before being partial or totally vaporized by the EDC lean solvent feedwhich is sent through heat exchanger 286. Operations of the Pre-DC 281for concentrating the fermentation broth are the same as those describedfor Pre-DC 261 in FIG. 4.

A vaporous side-cut stream containing 80 to 95 wt % ethanol, preferably85 to 92 wt % ethanol, and more preferably 90 to 92 wt % ethanol iswithdrawn from Pre-DC 281 from a location in Pre-DC 281 that ispreferably 1 or 2 trays below the top and just below the liquid refluxentry point and the vapor fed into EDC 282 as the vaporous ethanol feed.The overhead stream in line 113 comprising vaporous ethanol, water andessentially all the light minor components including formaldehyde,methanol, and light esters from the top of Pre-DC 281 passes throughpartial condenser 285, which is maintained at an average temperaturefrom 60 to 65° C. At this temperature range, only ethanol and water arecondensed and the condensate from accumulator 289 is recycled with pump292 into Pre-DC 281 as liquid reflux via line 114. The vaporous lightcomponents, which are not suitable for human consumption, exit condenser285 via line 116 and are subsequently condensed separately at lowertemperatures.

The bottoms stream from Pre-DC 281 which contains essentially water iswithdrawn via line 117 by pump 298. A portion of this bottoms stream isheated in reboiler 295 and recycled into the bottom of Pre-DC 281 vialine 118 whereas the remaining portion of the bottom stream is passedthrough heat exchanger 284 to heat the aqueous ethanol feed to Pre-DC281 and the cooled bottoms stream is sent back to a fermentation section(now shown) via line 119. By designing the process so that the side-cutat the top of Pre-DC 281 contains no more than 92 wt % ethanol, theliquid reflux and number of trays in Pre-DC 281 are substantiallyreduced.

A vaporous side-cut from Pre-DC 281 is fed via line 115 to the middleportion of EDC 282 whereas lean solvent from the bottom of SRC 283 isfed by pump 300 via lines 130 and 120 to the upper portion of EDC 282after being cooled in heat exchanger 286. The lean solvent entry pointin EDC 282 is selected so that there are a few trays above the solventtray to essentially eliminate the presence of entrained solvent, if any,from being carried over to the overhead of EDC 282.

Again, no liquid reflux is recycled back to the top of EDC 282 althougha reflux-to-distillate ratio of 0 to less than 0.5 can be used fornon-functional purposes, such as for knocking down any entrained solventfrom the overhead vapor stream. Water vapor containing less than 20 wt %and preferably less than 5 wt % of the ethanol in the EDC feed exits thetop of EDC 282 via line 121 and is condensed in heat exchanger 287. Thecondensate is transferred to accumulator 290 and recycled by pump 293via lines 122 and 112 to the middle portion of Pre-DC 281 to recover theethanol. The rich solvent, which contains substantially pure ethanol andthe solvent, is withdrawn from the bottom of EDC 282 and is fed vialines 123 and 125 by pump 299 as a partially vaporized mixture into themiddle portion of SRC 283. A portion of the EDC bottoms is heatedthrough reboiler 296 and recycled to the bottom of EDC 282 via line 124to maintain the vapor flow in EDC 282.

Overhead vapor stream from SRC 283, which contains at least 99.5 wt %ethanol, is transferred via line 126 to condenser 288 under reducedpressure (vacuum), where the condensate is transferred to accumulator291 via line 127. SRC 283 overhead is connected to a vacuum source (notshown) such as a vacuum pump or steam injector through line 132. Atleast a portion of the condensate is recycled to SRC 283 as liquidreflux by pump 294 via line 129. The remaining portion of the condensateis removed via line 128 in the form of anhydrous ethanol. Lean solventis removed by pump 300 from the bottoms of SRC 283 and a portion of thelean solvent is heated in reboiler 297 and recycled to the bottom of SRC283 via line 131 to maintain the bottoms temperature in the range of 160to 200° C. and preferably in the range of 160 to 180° C.

Other non-limiting operating conditions for EDC 262 and SRC 263described for FIG. 4, such as temperature, pressure and stream flow are,respectively, adjusted for the operations of EDC 282 and SRC 283 asdepicted in FIG. 5.

EXAMPLES

The following examples are presented to further illustrate the preferredembodiments of this invention and are not to be considered as limitingthe scope of this invention.

Example 1

This example demonstrates that anhydrous ethanol with 99.5 wt % purityand no solvent contamination can be produced from an aqueous ethanolfeed in a continuous EDC operating without liquid reflux at the top ofthe column. Pilot plant tests were carried out in a 3-inch (7.62 cm)diameter continuous EDC which consisted of nine (9) structurally packedsections with a chimney tray at the top of the column. It was estimatedthat there were four theoretical trays between the lean solvent and theethanol feed entry points.

Aqueous feed containing 90.82 wt % ethanol and 9.18 wt % water waspre-heated to 70 to 80° C. and fed into the middle portion of the EDC(4^(th) section from the bottom). A lean solvent containing 99.50 wt %glycerin and 0.50 wt % water was fed to the chimney tray at the top ofthe column after pre-heating. The rates of ethanol feed and glycerinsolvent feed were 1.94 and 7.39 Kg/h, respectively (the overallsolvent-to-feed (S/F) weight ratio was 3.8). Allowing 0.5 wt % water inthe lean glycerin feed can significantly reduce the severity of the SRCoperation. The EDC kettle temperature was maintained at 149° C. togenerate a vapor stream in the EDC. Surprisingly, the EDC columntemperature profile was within the range of 80 to 90° C. even though theboiling point of glycerin is 290° C. Even without liquid reflux at thetop of the EDC, anhydrous ethanol containing 99.52 wt % ethanol, 0.48 wt% water, and no measurable levels of glycerin was produced from the EDCoverhead, at a rate of 1.72 Kg/h. Rich solvent containing 96.41 wt %glycerin, 1.12 wt % ethanol, and 2.37 wt % water was withdrawn from theEDC bottom at a rate of 7.79 Kg/h. Based on experimental data, ethanolrecovery was determined to be 96.2 wt %. Higher ethanol recovery can bereadily achieved by using an EDC with a larger number of separationstages.

These pilot plant results demonstrate that, even when using an EDC withrelatively few separation stages and operating with no liquid reflux atthe top the column, a glycerin/water solvent can break the azeotrope ofethanol and water and produce anhydrous ethanol with very high purity(99.5 wt %) and no solvent contamination.

Example 2

This example demonstrates that significant energy can be saved byeliminating liquid reflux at the top of an EDC without sacrificinganhydrous ethanol purity or product recovery.

Approximately 100 kg/h of aqueous ethanol at 25° C. was fed to tray 12(counting from the top) and 350 kg/h of glycerin solvent at 45° C. wasintroduced to tray 3 of an EDC consisting of 13 theoretical trays(excluding the kettle). The aqueous ethanol feed contained 90 wt %ethanol and 10 wt % water and the glycerin solvent feed contained 99.7wt % glycerin and 0.3 wt % water. The EDC was operated with no liquidreflux and alternatively with a liquid reflux ratio (R/D) of 1.0 tocompare the energy requirements of the EDC. In both cases, the columntemperature increased from 78° C. at the top (tray 1) to 128° C. at thebottom (tray 13), while the column pressure increased from 1.00 to 1.25atmospheres through the top to the bottom. The EDC kettle operated at175° C. and 1.27 atmospheres.

Under R/D of 1.0 the energy requirement of the EDC was determined to be2,865 KJ/L of produced anhydrous ethanol from the overhead of the EDC.Purity and recovery of the anhydrous ethanol were 99.7 wt % and 99.99 wt%, respectively. No detectable glycerin was found in the ethanolproduct. Under similar operating conditions for the EDC, it was foundthat anhydrous ethanol with 99.7 wt % purity and 99.9 wt % recoverycould be produced from the overhead, without employing any liquid refluxnear the top of EDC. Again, no detectable glycerin was found in theethanol product. However, the energy requirement without the liquidreflux was significantly lower at 2,234 KJ/L (a 22% reduction).

Example 3

This ED process simulation result demonstrates that significant energycan be saved by employing partial ethanol recovery in an EDC andrecovering the lost ethanol in a front-end pre-distillation (beer)column. The process simulator was upgraded and validated by theexperimental data from an ED commercial demonstration plant.

Specifically, a small fraction of ethanol in ethanol feed stream to theEDC is allowed to be lost to the bottom (rich solvent) stream of theEDC. As shown in FIG. 4, the lost ethanol is then recycled and recoveredin a front-end pre-distillation column where the VLE curve ofethanol/water is extremely favorable for distillation. This novelprocess configuration not only reduces the overall process energyrequirements, but also allows the EDC to be operated under relativelymild conditions.

In accordance with the process shown to FIG. 4, 12,600 kg/h offermentation broth containing 12 wt % ethanol and 88 wt % water isintroduced into the process via line 81 and combined with a smallrecycled stream from SRC 263 (via line 100) before entering the middleportion of Pre-DC (PDC) 261 via line 82. The side-cut (just below thePDC reflux tray) containing approximately 90 wt % ethanol is fed to themiddle portion of EDC 262 via line 85, while the lean solvent containing99.7 wt % ethylene glycol (EG) and 0.3 wt % water is fed to near the topof EDC 262 via line 90. Solvent-to-feed weight ratio (S/F) in EDC 262 ismaintained at 3.0. The EDC is operated under conditions with no ethanolloss (essentially 100% recovery) and with 2.5 wt % ethanol loss, todetermine the difference in overall process energy requirements for thesystem which includes the PDC, EDC, and SRC process steps. The processparameters are summarized in Table 1.

TABLE 1 1. Feed Flow Rate and Composition Ethanol feed rate to PDC:12,600 kg/h Composition of ethanol feed to PDC: 12 wt % ethanol and 88wt % water Composition of ethanol feed to EDC: 90 wt % ethanol and 10 wt% water Composition of solvent feed to EDC: 99.7 wt % EG and 0.3 wt %water Solvent-to-feed weight ratio in EDC (S/F):   3.0 2. Product FlowRate and Composition Ethanol product rate from EDC overhead: 1,512.45kg/h  Composition of ethanol product from EDC overhead: 99.6 wt %ethanol and 0.4 wt % water Water rate from the process: 11,088 kg/hComposition of water from the process: 99.95 wt % water and 0.05 wt %ethanol 3. Number of Theoretical Stages of the Column Pre-DistillationColumn (PDC): 50 Extractive Distillation Column (EDC): 15 SolventRecovery Column (SRC):  9 4. Column Operating Conditions 0% Ethanol Lossin EDC 2.5% Ethanol Loss in EDC PDC EDC SRC PDC EDC SRC Liquid Reflux(R/D) 1.81 1.66 1.81 1.75 0.58 1.50 Kettle Temperature (° C.) 131.6185.2 163.5 131.6 179.6 163.4 Overhead Presure (atm) 2.2 1.2 0.26 2.21.2 0.26 Bottom Pressure (atm) 2.8 1.8 0.40 2.8 1.8 0.40 Reboiler Duty(KJ/sec) 1,348 905 190 1,355 479 235 Total Process Energy (KJ/h)8,794,800 7,448,400 Unit Product Energy (KJ/L) 4,565 3,867

The data presented in Table 1 show that about a 15.3% reduction inenergy of the integrated process consisting of PDC, EDC, and SRC can beachieved by lowering the ethanol recovery in the EDC to 97.5 wt % andrecovering the lost 2.5 wt % ethanol in the front-end pre-distillation(beer) column. The unit product energy for producing anhydrous ethanolis reduced from 4,565 to 3,867 KJ/L. The reduction of ethanol recoveryin the EDC also leads to a reduction in the liquid reflux in the Pre-DC,EDC, and SRC from 1.81 to 1.75, 1.66 to 0.58, and 1.81 to 1.50,respectively.

Example 4

The following comparison shows that the inventive process issignificantly more energy efficient than any of the current commercialmethods of anhydrous ethanol production.

Some early studies concluded that azeotropic distillation to be moreenergy efficient than extraction distillation. For example, Black et al,“Extractive and Azeotropic Distillation,” Am. Chem. Soc. Advances inChemistry Series No. 115, 1-16 (1972), produced anhydrous ethanol froman aqueous ethanol feed by extractive distillation using ethylene glycolas the solvent and by azeotropic distillation using n-pentane as theentrainer. In this comparison, the RID ratio of the extractivedistillation column was set at 1.8, which is equivalent to areflux-to-feed ratio of 1.55 as described in the article. Black et alconcluded that azeotropic distillation was more energy efficient.

This conclusion is flawed because the investigators designed theextractive distillation column with an exceedingly high R/D ratio of 1.8and charged the column with an aqueous ethanol feed that contained anunnecessarily high ethanol concentration of 85.64 mole % or 93.84 wt %.Although azeotropic distillation requires a feed with a composition thatis near that of an azeotropic mixture of ethanol and water, i.e., 95.6wt % ethanol, extractive distillation has virtually no restrictions onthe feed composition. This important difference means that much moreenergy is expended with azeotropic distillation in the pre-distillationprocess. As evidenced by the data in FIG. 1 of U.S. Pat. No. 4,559,109to Lee et al, in concentrating fermentation broth with 10-12 wt %ethanol to the azeotrope level of 95.6 wt %, the first stage of raisingthe concentration from 10-12 wt % to 92 wt % consumes only 47% of thetotal distillation energy whereas the final stage of raising theconcentration from 92 to 95.6 wt % consumes 53% of the total energy.

It is apparent that Black et al. did not recognized that byconcentrating the feed stock to only 90 wt %, instead of 93.8 wt % andby operating the extractive distillation column with minimal or noliquid reflux, extractive distillation is clearly more economical thanazeotropric distillation. An estimate of the energy requirements forproducing anhydrous ethanol from fermentation broth confirms thesignificant advantages of using extraction distillation where the feedstock has a lower ethanol concentration over competing processes thatemploy a similar feed stock with a higher ethanol concentration. Theresults are summarized in Table 2.

TABLE 2 Comparison of Energy Consumption of Leading Ethanol PurificationTechnologies (Unit: KJ/Liter of Product) Distillation from Dehy- TotalTechnology 10 to 95 vol % dration Energy Azeotropic Distillation 3,6892,573 6,262*  PSA (Molecular Sieves) 3,689 1,000 4,689**  Inventive EDProcess 2,532 (to 90 vol %) 1,335 3,867*** In all cases, vapor fromoverhead of the pre-ditillation column is fed to the dehydration unitdirectly without condensing to save the vaporization energy. *Values aretaken from the SRI report. **Values are estimated from published data ofthe most advanced pressure swing adsorption (PSA) process. ***Energyrequirement for the ED process with R/D of 0.58 and partial ethanolrecovery (97.5 wt %) in the EDC, which can be further reduced when R/Dis decreased to zero.

In this comparison, energy consumptions were estimated for azeotropicdistillation and adsorptive techniques, i.e., molecular sieves. In theseconventional processes, a pre-distillation step raises the ethanolconcentration from 10 to 95 vol % and, thereafter, a dehydration stepcompletes the process to yield the anhydrous ethanol. For the inventiveextractive distillation process, pre-distillation requires less energysince it raises the ethanol concentration to only 90 vol %. In thisexample, the calculation is based on a design whereby the R/D ratio inthe extractive distillation column is 0.58. The energy requirement canbe substantially lowered by employing less or no reflux. Even with R/Dratio of 0.58, the ED process of this invention requires the lowestenergy among all major commercial technologies, just 3,867 KJ/L.

The foregoing has described the principles, preferred embodiment andmodes of operation of the present invention. However, the inventionshould not be construed as limited to the particular embodimentsdiscussed. Instead, the above-described embodiments should be regardedas illustrative rather than restrictive, and it should be appreciatedthat variations may be made in those embodiments by workers skilled inthe art without departing from the scope of present invention as definedby the following claims.

1. An improved extractive distillation (ED) process for dehydrating anaqueous feedstock containing ethanol and water that comprises the stepsof: (a) introducing an aqueous feedstock comprising ethanol and waterinto a middle portion of an extractive distillation column (EDC); (b)introducing a high-boiling water selective, salt-free solvent into anupper portion of the EDC to contact the aqueous feedstock underextractive distillation conditions to produce a liquid bottoms streamthat comprises water and high-boiling water selective solvent and toproduce a vaporous overhead stream that comprises greater than 99.5weight percent ethanol, wherein the EDC is operated with no liquidreflux at the top of the EDC; (c) withdrawing at least a portion of thevaporous overhead from the EDC as a purified ethanol product; (d)feeding at least a portion of the liquid bottoms stream of the EDC intoa solvent recovery column (SRC) to remove water therefrom and to yield alean high-boiling water selective solvent stream; and (e) recycling atleast a portion of the lean high-boiling water selective solvent streaminto the upper portion of the EDC.
 2. The process of claim 1 wherein theaqueous feedstock, prior to being introduced into the EDC, is initiallypreheated by the vaporous overhead stream and subsequently preheated bythe lean high-boiling water selective solvent stream that is recycled tothe upper portion of the EDC.
 3. The process of claim 1 wherein theaqueous feedstock comprises 85 to 95 wt % ethanol.
 4. The process ofclaim 3 wherein the aqueous feedstock comprises 90 to 92 wt % ethanol.5. The process of claim 1 wherein the high-boiling water selective,salt-free solvent is selected from the group consisting of glycerin,tetraethylene glycol, ethylene glycol, diethylene glycol, and mixturesthereof.
 6. The process of claim 5 wherein the high-boiling waterselective, salt-free solvent is glycerin and the purified ethanolproduct is food-grade ethanol.
 7. The process of claim 1 wherein step(b) comprises introducing a high-boiling water selective, salt-freesolvent through an entry point at the upper portion of the EDC whereinthe temperature of the high-boiling water selective solvent is about 10°C. to 20° C. lower than that of the EDC at the entry point in order togenerate internal reflux.
 8. The process of claim 1 wherein the leanhigh-boiling water selective solvent which is removed from a bottom ofthe SRC has less than about 0.5 wt % water.
 9. The process of claim 1wherein the EDC has a bottoms temperature in a range of 160° C. to 200°C.
 10. The process of claim 1 wherein the SRC has a bottoms temperaturein a range of 160° C. to 200° C.
 11. The process of claim 1 wherein theSRC has an operating pressure in a range of 50 to 500 mmHg (absolute).12. The process of claim 11 wherein the SRC has an operating pressure ina range of 150 to 300 mmHg (absolute).
 13. The process of claim 1wherein the amount of ethanol that is in the liquid bottoms stream ofthe EDC is less than about 0.5 wt % of the amount of ethanol that is inthe aqueous feedstock.
 14. The process of claim 1 wherein step (a)comprises forming a liquid bottoms stream comprising water, ethanol, andhigh-boiling water selective solvent and in step (d) the SRC produces anoverhead stream comprising water and ethanol that is condensed to form acondensate that is fed into a distillation column to yield a liquidbottoms stream consisting essentially of water and an overhead streamcomprising ethanol and water which is recycled to the EDC.
 15. Theprocess of claim 14 wherein the amount of ethanol that is in the liquidbottoms stream of the EDC is about 20 wt % of the amount of ethanol thatis in the aqueous feedstock.
 16. The process of claim 14 wherein theamount of ethanol that is in liquid bottoms stream of the EDC is no morethan about 5 wt % of the amount of ethanol that is in the aqueousfeedstock.
 17. An improved extractive distillation (ED) process fordehydrating an aqueous feedstock containing ethanol and water thatcomprises the steps of: (a) introducing an aqueous feedstock comprisingethanol and water into a middle portion of an extractive distillationcolumn (EDC); (b) introducing a high-boiling ethanol selective,salt-free solvent into an upper portion of the EDC to contact theaqueous feedstock under extractive distillation conditions to produce aliquid bottoms stream consisting essentially of ethanol and high-boilingethanol selective solvent and a vaporous overhead stream consistingessentially of water, wherein the EDC is operated with no liquid refluxat the top of the column; (c) feeding at least a portion of the liquidbottoms stream of the EDC into a solvent recovery column (SRC) to removeethanol therefrom and to yield a bottoms stream that comprises leanhigh-boiling ethanol selective solvent and an overhead stream whereby atleast a portion of the overhead is withdrawn as a purified ethanolproduct with higher than 99.5 wt % purity; and (d) recycling at least aportion of the lean high-boiling ethanol selective solvent from thebottoms stream of the SRC into the upper portion of the EDC.
 18. Theprocess of claim 17 wherein the aqueous feedstock, prior to beingintroduced into the EDC, is initially preheated by the vaporous overheadvapor stream and subsequently preheated by lean high-boiling ethanolselective solvent that is recycled to the upper portion of the EDC. 19.The process of claim 17 wherein the aqueous feedstock comprises 85 to 95wt % ethanol.
 20. The process of claim 19 wherein the aqueous feedstockcomprises 90 to 92 wt % ethanol.
 21. The process of claim 17 wherein thehigh-boiling ethanol selective, salt-free solvent is selected from thegroup consisting of 2-phenyl-phenol, cumyl phenol, diisopropyl phenol,cyclohexyl cyclohexanone, cyclohexyl cyclohexanol, methyl benzoate,dipropylene glycol dibenzoate, trimellitic anhydride, and mixturesthereof.
 22. The process of claim 17 wherein step (b) comprisesintroducing a high-boiling ethanol selective, salt-free solvent throughan entry point at the upper portion of the EDC wherein the temperatureof the high-boiling ethanol selective solvent is about 10° C. to 20° C.lower than that of the EDC at the entry point in order to generateinternal reflux.
 23. The process of claim 17 wherein the amount ofethanol that is in the EDC overhead stream is less than about 0.5 wt %of the amount of ethanol that is in the aqueous feedstock.
 24. Theprocess of claim 17 wherein step (a) comprises producing a vaporousoverhead stream comprising water and ethanol and the process furthercomprises the steps of: (e) feeding at least a portion of the vaporousoverhead stream from the EDC into a distillation column to produce asecond liquid bottoms stream consisting essentially of water and asecond overhead stream comprising ethanol and water; and (f) recyclingthe second overhead stream into the EDC.
 25. The process of claim 24wherein the amount of ethanol that is in the vaporous overhead stream ofthe EDC is about 20 wt % of the amount of ethanol that is in the aqueousfeedstock.
 26. The process of claim 24 wherein the amount of ethanol inthe vaporous overhead stream of the EDC is no more than about 5 wt % ofthe amount of ethanol that is in the aqueous feedstock.